Nemo 33 is the deepest indoor swimming pool in the world. The pool is located in Brussels, Belgium. Its maximum depth is 34.5 metres (113 ft). It contains 2,500,000 litres (550,000 imp gal; 660,000 US gal) of non-chlorinated, highly filtered spring water maintained at 30 °C (86 °F) and holds several simulated underwater caves at the 10 metres (33 ft) depth level. Due to the warm temperature in the pool, divers can dive for extended periods without a wet suit. There are numerous underwater windows that allow outside visitors to look into the pools at various depths.